Over the past 25 years, metal fasteners have been used extensively in the construction of aircraft. The fasteners are commonly used to fasten two sections of aircraft skin together. These fasteners are often similar in appearance to commercial bolts or rivets. Typically, metal fasteners have a head, and a shank. The shank is commonly threaded so that a nut can be affixed to the shank whereby the two sections of aircraft skin are fastened between the head and the nut. It is also common to manufacture metal fasteners without threads in the shank, and with this configuration the end of the shank is pressed on the other side of the aircraft skin to form a head which fastens the two sections together.
During normal operation, fasteners used in aerospace applications are generally placed under a high shear load, and a relatively low tension load. Thus, it is desireable to design fasteners which exhibit a high shear strength, while the requirements for tensile strength are not as rigorous. Because of their high shear strength, these types of fasteners are commonly known as shear pins. Many of the most satisfactory metal shear pins are made from 6AL4V titanium which has a design shear strength of 95 KSI.
The use of composite materials to make certain components of aerospace vehicles has increased rapidly over the past 20 years. Numerous metal shear pins have been developed to join together the composite components of these vehicles. The use of composite shear pins to replace metal shear pins in such applications results in the elimination of corrosion, as well as considerable weight savings. In addition to these advantages, a composite shear pin improves the match between the coefficient of expansion of the fastener and the structure.
Composite shear pins have been fabricated from a fiber-reinforced thermoplastic matrix. Initially, a panel is made by stacking composite tapes one on top of the other. The tapes are heated and pressed which causes the matrix material to melt, consolidate, and then, upon cooling, to become a flat solid panel. The panel is then cut into square section rods. These square section rods advantageously have a thickness equal to or slightly greater than the desired shear pin diameter. The square section rods are then ground or machined into round cross-section rods which are then cut to length and warm-formed by a suitable method to produce a manufactured head and also threads, if these are desired.
Recently, some shear pins made from fiber-reinforced polymer matrix composites have been developed that provide average shear strengths of about 50 KSI (50,000 PSI). These composite shear pins are finding applications, for the reasons previously mentioned, in joints where titanium shear pins are presently used, but where much less than 95 KSI loads are seen by the shear pins concerned. While 50 KSI average shear strength is satisfactory in many applications, higher shear strengths which are closer to the 95 KSI of 6AL4V titanium are desirable to make substitutions possible in more highly loaded joints. Thus, a need exists for a composite fastener which can be used in applications where the expected shear load is in excess of 50 KSI.